Guitar Pro natively supports exporting of its own projects to MIDI (.mid, .midi) or PCM audio (.wav, .wave). The easiest way would be to get WAV file and use some of the wav to mp3 converters to create MP3 audio from it. However, this will still be instrumental music, just compressed to reduce the file size.
Open GP4 file Open MP3 fileA MIDI Converter designed to easily convert midi files